Dispersing fine grained powder having an average primary particle size of micron to nanometer scales or smaller into a solvent, resin, or polymer has become increasingly important in producing a variety of useful materials, such as functional films, adhesives, coatings, bulk materials, and the like. In particular, dispersing fine-grained powder is important in terms of forming uniformly distributed composite materials without defects, aggregation, or liquid handling difficulties (liquid viscosity reduction). Increasingly important are processes which create concentrated particle dispersions, often called “concentrates” or “particle pastes”.
A number of methods of dispersing fine grained powder into a quasi-stable solutions are known. For example, roller mills and other rotating types of mixers disperse particles by a crushing agglomerated particles by applying a low speed, high tangential shear force to the particles. The roller mill method uses compressive forces to crush fine grained powder to smaller particle sizes, while at the same time absorbing resin components such as a compatibilizing agents are compressed into the surface of the particle by a high shearing mixing force, but non-uniform mixed portions are also compressed by this strong force causing the formation of larger (agglomerated) particles. Particles smaller than the average interstitial space can have little compressive force applied to them by the process. This not only prevents crushing of these smaller particles but also increases throughput time consequently reducing productivity. Roller mills also often are acoustically polluting, cause low frequency vibrations, and are labor intensive.
Homogenizing high speed/high torque mixers are known to crush particles by high speed rotation in the relatively narrow rotor/stator gap. Such processes undesirably form sediment when fine grained powder having an average primary particle size in nanometer to micron scales are dispersed into a low viscosity solvent or resin. Some believe that the attractive particle energy is low and the mixing vortex provides insufficient collision frequency of agglomerated particles, thereby preventing the facile formation of a dispersion liquid with reduced particle size from the starting fluid, and a less homogeneous dispersion overall.
Jet mill dispersion methods are known to cause collisions of liquids at high pressure, which requires large, dangerous, and expensive equipment. The drawbacks of these methods are similar to the homogenizing high speed mixing systems. This dispersion system also has drawbacks in terms of cleaning and maintenance do to complicated internal mechanisms. Jet mills also often are large, expensive, and changing compositions or cleaning are very labor intensive.
Small diameter bead or sand mill dispersion methods are known for uniformly dispersing a solution containing fine grained powder via the coordinated use of bead diameter, filling factor of beads into a device, residence time of the dispersed liquid in the device, particle/volume ratio of the dispersion, and an absorption state of a dispersion agent or other additive to the particle surface. However, particle crushing occurs during the process, which can give rise to agglomeration of particles and increase liquid viscosity, which reduces productivity. Dispersions made by the bead mill dispersion method are also limited by the relatively large size of commercially available beads and the interstitial space between the crushing beads (determined by bead diameter. Thus, it is difficult to disperse powder having an average primary particle size in micron to nanometer scales or less into a solvent or a solution, and to construct a dispersion system that provides high volume production. The beads themselves are often very difficult to clean since the particle used can be highly compressed into the surface structures of the bead themselves. Cleaning often requires extensive flushing with water or solvent, which is expensive, potentially polluting, and highly labor intensive.
Ultrasonic batch type dispersers are known for preparing a small amount of liquid such as for pretreatment of measurement of an experiment level or a particle size distribution. Ultrasonic batch type dispersers generally have liquid circulation problems, and as a result, ultrasound is not uniformly applied to liquid dispersions. Ultrasonic dispersers may also damage delicate materials.
Various problems, such as those described above, become more pronounced as the particle size of the fine grained powder becomes smaller. Moreover, dissolved particles generally cannot be dispersed by any of the previously mentioned methods. The aforementioned techniques also suffer from particle agglomeration problems. Because the surface area/volume ratio of fine grained powders increases with decreasing particle size, there is an increased tendency for particles to agglomerate, which is generally undesirable. Reducing the size of these so-called “secondary agglomerated particles” is difficult for commercial powder producers. Accordingly, there is a continuing need to disperse sub-micron scale (ca. 0.1 to 1 microns) and nanometer-scale (ca. 0.1 to about 100 nanometers) particles in dispersing media.